1. Field of the Invention
The present invention relates to a method for improving golf ball covers. More specifically, the present invention relates to a method for improving the cover shear durability of golf ball covers.
2. Description of the Related Art
When used in golf ball cover stock, injection moldable thermoplastic polyurethanes offer significant economic advantages over thermoset polyurethanes and polyureas. They can also be formulated to exhibit attractive properties with good “feel”, controllability, and rebound resilience. Several methods have been used to improve the physical property profile and groove shear durability (i.e. resistance to scuffing, cutting, and tearing) of thermoplastic polyurethane (“TPU”) golf ball covers. Some include incorporating polyisocyanates into the TPU cover layer either before or during injection molding in an effort to impart chemical cross-linking. For example, U.S. Pat. No. 6,663,507 and US Patent Applications 2008/0161134 and 2008/0207846 describe dry blending polyisocyanate masterbatches with TPU prior to injection molding. Disadvantages of this prior art include low polyisocyanate reactivity, conversion, cross-linking, and thus durability improvement. Conversely, high polyisocyanate reactivity in the melt can cause TPU molecular weight loss with diisocyanates and rapid molecular weight growth with triisocyanates and higher order polyisocyanates. In both cases, the resulting changes in melt viscosity limit processability, especially in the context of thin wall injection molding.
The prior art is lacking in a method that results in the use of polyisocyanates with reduced reactivity which minimizes polyisocyanate conversion in the melt and effectively decouples the molding process from polyisocyanate cross-linking, hence allowing increased processability. It has been discovered that exposing TPU cover layers to a catalyst solution following injection molding maximizes polyisocyanate conversion and thus durability.